This invention relates to logic circuits, and more particularly, to redundancy circuitry for repairing logic circuits containing defects.
Logic circuits such as programmable logic devices are devices that may be programmed to perform custom logic functions. Integrated circuit fabrication techniques are not perfect, so occasionally a programmable logic device or other logic circuit is fabricated with a defect. Unless the defect can be corrected before the logic circuit is put into use, the logic circuit must be discarded. Discarding such circuits is wasteful, particularly when the defects are relatively minor.
As a result, various redundancy schemes have been developed which allow spare circuitry to be switched into place to repair a defective portion of a circuit. In programmable logic devices, such redundancy schemes often involve the programming of multiplexer switches to redirect signals to a spare region of logic rather than the defective region of logic.
To reduce the amount of overhead required to provide redundancy, logic is often replaced in fairly large blocks. For example, in programmable logic devices that contain logic arranged in rows and columns, an entire row or column of logic may be replaced, even if a defect only affects a portion of the row or column.
Although such redundancy schemes can be implemented using a relatively small amount of circuitry, it is sometimes wasteful of logic resources to replace such large regions of logic at a time. This is a particular concern as integrated circuits increase in size.
It is therefore an object of the present invention to provide improved redundancy arrangements.
It is a further object of the present invention to provide redundancy arrangements that are satisfactory for repairing large integrated circuits.